Visem Technodag 18.06.2013 Radiation Curing of Coatings: From Traditional UV Curing to Prospective Coating Applications with NIR- Photopolymers Christian Schmitz
Nov 08, 2015
Visem Technodag 18.06.2013
Radiation Curing of Coatings:
From Traditional UV Curing to Prospective
Coating Applications with NIR-
Photopolymers
Christian Schmitz
visible
wavelength
ultraviolet UV-C / UV-B / UV-A
(near)infrared (N)IR
Radiation Curing
380nm 780nm
coated substrate
light source: mercury arc lamp
- fast curing by high-intensive, UV emitting light sources
- reduce of organic solvents
environmentally-friendly technology
UV-Curing vs. Thermal Curing
- solvent
- resin
- crosslinking of resin and reactive solvents
- no evaporating of reactive solvents
UV
heat
- solvents reduce viscosity
- thermal-induced crosslinking of resin
- simultaneous evaporating of the solvents
UV-Curing via Radical and/or Cationic
Polymerization I - Initiator R - reactive resin/solvent
I UV-radiation
generate high-reactive species
I R
growing by chain reaction
I R R R ...
- UV-radiation absorption of initiator and photochemistry generates radicals/cations
- increase of network density by chain reaction
- solidifies coating resulting in film-formation --> fast proceeding in every step --> short curing period
Applications of UV-Curing
Advantages specify the field of application - printing-ink - wood coating - powder coating --> economic aspect
--> heat sensitve substrate; high output
*www.kem.de/konstruktion/-/article/30599594/31436464/maximized/
More and more applications
are going to switch to these
energy-saving technologies!
Limitations of UV-Curing-Systems
- curing is influenced by thickness of coating --> loss of adhesion
- critical for depth-curing particular if system contains UV-absorbing components
- optimized OD for a system with no bleaching: 0.43
- inhibition of curing process by atmospheric oxygen
- need of safety installations --> risk for operator (eyes, skin) caused by UV-radiation
radiation
coating layer
Selection of Light Sources
- emission must fit the absorbance of the initiator
- energy-efficiency by maximal overlapping
Research HS-Niederrhein: NIR-
Photopolymerization and new Light Sources
visible
wavelength
ultraviolet UV-C / UV-B / UV-A
(near)infrared (N)IR
380nm
- (n)ir-radiation well-known as thermal-energy source
- shift of the absorption --> solve problems of competing absorption in the UV-range
- initiatorsystem consists of two components
780nm
sensitizer absorbing component
coinitiator generates radicals
UV-curing vs. NIR-Curing
UV NIR
- high sensitivity
- established curing system
- yellow, red and blue pigmented coatings
- curing of layers upto several mm in just one step
- NIR-light uncritical in handling
- problematic with pigments or light stabilizers
- harmful UV-light
- requires safety light shielding
- unknown to coating-technology
- less sensitive than UV-systems
Research HS-Niederrhein: NIR-
Photopolymerization and New Light Sources
NIR-LED arrays
- small range of emission suitable to the sensitizer
- low intensity
- requires development of more intensive LEDs in different wavelengths including UV, visible and NIR
IR-radiant heater
- wide range of emission
- small amount of usable emission
Diode laser
- laser spot emission needs modification for exposure of large areas
- x,y-modulation of the emitter
- homogenous laserlines *Beier; Laser Technik Journal Vol 8, No.2 (2011) *Homburg et al.; Proc. of SPIE Vol 6216 (2006)
Laser technology as NIR Light Sources
x,y modulated laser or objekt
diode laser bars
line generator
NIR-Photopolymerization in Powder Coating
Technology
- first experiments with NIR-polymerizable powder coatings (at 110C)
- state of the arts: two-step curing process
- diode laser for dual process technology --> melting by energy absorption and curing process simultaneously
melting process
< 120C application radiation curing
Thank you for your attention!
Acknowledgement
- BMWi (ZIM cooperation project, grant # KF2914003BN2)
- University of Applied Sciences (start up funding)
- County of NRW (scientific equipment program)
- FEW Chemicals GmbH (supplying of NIR sensitizers)